Monitoring power-related parameters in a power distribution unit

ABSTRACT

A power distribution unit (PDU) disposable in an electrical equipment rack. The PDU has a housing, a power input penetrating the housing, outlets in the housing, a processor disposed in the housing, voltage and current sensors, and a voltage calculation procedure communicable with the processor. The processor samples voltage and current waveforms and calculates RMS values and other power parameters. A method of managing electrical loads each drawing electrical power from a PDU includes repeatedly sampling voltage across and current flowing through each of the loads, calculating raw RMS values of voltage and current, and scaling the raw RMS values to obtain corrected RMS voltage and current values and other power parameters.

This patent application claims priority from U.S. Provisional PatentApplication 61/157,546, filed 4 Mar. 2009, titled “Systems and Methodsfor Monitoring Power-Related Parameters in a Power Distribution Unit”,the contents of which are incorporated herein by this reference.

BACKGROUND

Power monitoring and metering have long been used in some applicationsto provide any of a number of items of information to different entitiesthat supply, deliver, and consume power. One common use of suchinformation is to determine energy consumption for purposes of billing auser for the power received by that user. Centralized computingfacilities, often referred to as server farms or as data centers,continue to house more and more computing equipment. Such facilitiesoften have numerous individual pieces of computing equipment arranged inracks. Power Distribution Units (PDUs) have long been utilized to supplypower to racked electronic equipment in such facilities and in otherkinds of facilities as well.

A conventional PDU is an assembly of electrical outlets (also calledreceptacles) that receive electrical power from a source and distributethe electrical power to one or more separate electronic appliances. Eachsuch unit has a power cord plugged in to one of the outlets. PDUs alsohave power cords that can be directly hard wired to a power source ormay use a traditional plug and receptacle connection. PDUs are used inmany applications and settings such as, for example, in or on electronicequipment racks. A PDU located in an equipment rack or other cabinet,together with other devices connected to the PDU such as environmentalmonitors, temperature and humidity sensors, fuse modules, orcommunications modules that may external to or contained within the PDUhousing may be collectively referred to as a Cabinet Distribution Unit(CDU).

As mentioned, computing facilities generally include electronicequipment racks, such as standard RETMA racks, that commonly compriserectangular or box-shaped housings sometimes referred to as a cabinet ora rack and associated components for mounting equipment, associatedcommunications cables, and associated power distribution cables.Electronic equipment is commonly mountable in such racks so that thevarious electronic devices are aligned vertically one on top of theother in the rack. Often, multiple such racks are oriented side-by-side,with each containing numerous electronic components and havingsubstantial quantities of associated component wiring located bothwithin and outside of the area occupied by the racks. Such rackscommonly support equipment that is used in a computing network for anenterprise, referred to as an enterprise network.

In many cases, computing facilities such as server farms or data centerssupport large networks, referred to as enterprise networks. Enterprisenetworks exist to support large world-wide organizations and depend on acombination of technologies, e.g., data communications, inter-networkingequipment such as frame relay controllers, asynchronous transfer mode(ATM) switches, routers, integrated services digital network (ISDN)controllers, and application servers, and network management applicationsoftware. Such enterprise networks can be used to support a largecompany's branch offices or campuses throughout the world, and, as such,these networks have become mission critical to the functioning of suchorganizations. Masses of information are routinely expected to beexchanged, and such information exchanges are necessary to carry on thedaily business of modern organizations. For example, some internationalbanks have thousands of branch offices placed throughout Europe, Asiaand North America that each critically depend on their ability tocommunicate banking transactions quickly and efficiently with oneanother and with their respective headquarters.

A typical enterprise network uses building blocks of router and framerelay network appliances mounted in equipment racks. Such equipmentracks are distributed to remote point of presence (POP) locations in theparticular network. Each equipment rack can include frame relaycontrollers, routers, ISDN controllers, servers and modems, etc., eachof which are connected to one or more power sources. The value of POPequipment can range from $200,000 to $500,000, or higher, and the numberof individual devices can exceed a thousand.

Many equipment racks may be located in a data center. One or more suchdata centers may serve as data communication hubs for an enterprise. Onthe other hand, more than one enterprise may use computing facilities ina data center. Existing network management systems provide relativelylittle information representing the status of a data center or of racksor individual components in the center.

SUMMARY

Briefly and in general terms, aspects of the invention reside in a powerdistribution unit having a power input, a voltage sensor for the powerinput, a plurality of power outputs connectable to electrical loads, acurrent sensor for each output, and a power monitoring section. Thepower monitoring section receives signals from the sensors and samplesthe signals to obtain samples of voltage and current during a cycle ofinput power, calculates raw RMS values of voltage and current, andscales those values according to a predetermined calibration factor toobtain corrected RMS voltage and current values for one or more of theloads.

The power monitoring section may also calculate other power metrics forthe loads such as average active power, average apparent power, powerfactor, input current crest factor, phase angle between voltage andcurrent, and energy consumed over time. The energy consumed may beexpressed in kilowatt-hours.

In some embodiments the foregoing components are all installed in acabinet PDU housing.

In some embodiments the power monitoring section includes ananalog-to-digital converter (ADC) that receives signals from the sensorsand converts those signals to digital form, processing logic thatreceives the digital form of the signals, and a memory that may storethe converted ADC digital signals as well as any of the power metricsover one or more cycles of input power.

The processing logic may compare samples of voltage and current overdifferent cycles, or samples during a cycle may be compared withpre-stored model samples. In either comparison, if differences exceed apredetermined magnitude, an alert signal can be generated.

In some embodiments a timing sensor detects zero crossings of inputvoltage cycles. This information is used to calculate input voltagefrequency. The zero crossing information may also be used to calculate areload value that determines input power signal sampling intervals.

Power metrics and alert signals may be transmitted through acommunication section to a remotely-located network power manager.

A power distribution unit according to some embodiments includes a powerdistribution unit housing disposable in an electrical equipment rack ofthe type in which a plurality of electrical components are removablymountable. A power input penetrates the housing and a plurality of poweroutputs are disposed in the housing. A processor is also disposed in thehousing and is communicable with a voltage calculation procedure whichmay for example be an RMS calculation procedure. A voltage sensor iscommunicable with the power input and the processor, and a plurality ofcurrent sensors are communicable with one of the power outputs and withthe processor.

Aspects of the invention reside in a method of managing a plurality ofelectrical loads drawing power from a power distribution unit. Themethod includes sampling voltage across and current flowing through eachof the loads repeatedly during one cycle of input power, calculating rawRMS values of voltage and current from the samples, and scaling the rawRMS values according to a predetermined calibration factor to obtaincorrected RMS values.

The foregoing is a brief description of various aspects of variousexemplary embodiments. The scope of the invention is to be determined bythe claims as issued and not by whether given subject matter includesany or all such aspects, features, or advantages or addresses any or allof the issues discussed in the foregoing.

In addition, there are other advantages and varying novel features andaspects of differing embodiments. The foregoing and other features andadvantages will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Several embodiments including the preferred embodiments and currentlyknown best mode of the present invention are shown in the followingdescription and accompanying drawings in which:

FIG. 1 is a block diagram illustration of a system of an exemplaryembodiment of the present disclosure;

FIG. 2 is an illustration of an exemplary CDU embodiment;

FIG. 3 is a block diagram illustration of a system that senses currentand voltage on a per-outlet basis;

FIG. 4 is a schematic illustration of a bus circuit with voltageindicators of an exemplary embodiment;

FIG. 5 is a schematic illustration of a relay circuit of an exemplaryembodiment;

FIG. 6 is a schematic illustration of a current transformer conditioningcircuit of an exemplary embodiment;

FIG. 7 is a schematic illustration of a voltage sensing circuit of anexemplary embodiment;

FIG. 8 is a schematic illustration of a microcontroller and digitalportion of a relay drive circuit of an exemplary embodiment;

FIG. 9 is a block diagram illustration of a microcontroller of anexemplary embodiment;

FIG. 10 is a flowchart illustrating the collection and computation ofseveral parameters related to the monitored voltage and current of anexemplary embodiment;

FIG. 11 is a flowchart illustrating comparison of voltage and currentdata with historical or model data;

FIG. 12 is a flowchart illustrating summation of energy consumed overtime;

FIG. 13 is a flowchart illustrating use of power line frequency togenerate reload values for use in determining sampling intervals;

FIG. 14 is an illustration of a three printed circuit board PDU assemblyof an embodiment;

FIG. 15 is an illustration of a single printed circuit board PDUassembly of an embodiment;

FIG. 16 is a block diagram of a power management system embodyingaspects of the invention;

FIG. 17 is a schematic diagram of an embodiment that provides powermonitoring of an installation having a switch mode power supply; and

FIG. 18 is a pictorial representation of a power distribution unitaccording to an embodiment installed vertically in an equipment rack.

FIG. 19 is a partial cut-away view of an embodiment.

DETAILED DESCRIPTION

Exemplary embodiments of power distribution, monitoring, and managementsystems are described herein. The present disclosure provides exemplaryembodiments with capability to determine the power being delivered to apower distribution apparatus and the power being delivered from thepower distribution apparatus to one or more electrical loads. Thisfacilitates efficient determination of power usage for various differentcomponents that are associated with a facility, and therefore providesan ability to manage power to the various different components. In manycases, numerous PDUs and CDUs may be located in a facility, with eachsupplying power to several different electrical loads. Knowledge ofpower being delivered to various equipment in a facility may be used toevaluate, improve, and manage power consumption in a facility and acrossmultiple facilities, such as data centers.

In various embodiments, systems and methods are provided that sense andoutput information related to current and voltage provided to variousdifferent components and applications powered by one or more PDUs orCDUs. The current and voltage information may be used to provide any ofseveral measurements, referred to as power metrics, such as one or moreamong the following: power consumed by each component, power factor,crest factor, true RMS current and voltage measurements, active power,apparent power, and energy consumption.

One or more such metrics may be used for a number of purposes, such asfor analysis and actions that enhance the efficiency of components thatare receiving power, enhance the efficiency of a group of componentsthat are receiving power, enhance the efficiency of a data center, andenhance the efficiency of an enterprise network. Metrics also may beused, for example, to provide information related to management ofassets in a computing network, accurate tracking and billing of energyused by assets, and identification of components that are receiving orproviding power in an anomalous manner indicating a malfunction orpotential malfunction, to name a few examples.

Some embodiments of the present disclosure described in more detailbelow provide, alone or in combination, one or more advantages overtraditional PDUs. In certain embodiments, a PDU has an integrated ACinput clock solution, in which a power monitoring circuit or a powermeter does not require an external oscillator to provide a time base.Some embodiments may receive the incoming AC waveform to synchronize theinternal microprocessor clock to an AC cycle and provide power metricsthat are timed to AC cycles to provide accurate measurements. Such asolution can, in certain embodiments, further provide a relatively lowcost solution by reducing components, as well as a correspondingreduction in the area required and power used, for performing powermonitoring and metering.

Some embodiments include power monitoring and metering circuits that maybe used in a number of different applications, such as within a PDU,within a switched-mode power supply, and within commercial andresidential power meters, to name a few.

Some embodiments provide predictive failure of various power components.A digitized version of the AC power waveform of a component can be usedto identify anomalies in the waveform and thereby flag potentialproblems with the component. Also, the power being supplied to anindividual component can be monitored and used as a basis foridentifying abnormal operation of that component.

Some embodiments provide an accurate energy accumulation scheme for oneor more outputs associated with a single power monitoring and meteringcircuit. Voltage or current may be sampled during an AC cycle, and inthis regard in some embodiments both voltage and current are samplednearly simultaneously for an output.

In some embodiments, the product of each of the samples can be summedover the AC cycle. An AC cycle may be sampled at a pre-determinedfrequency. At each sample interval a single AC cycle is sampled at amuch higher sample rate (both voltage and current) to provide the powerinformation which is then accumulated at the lower waveform samplingfrequency to provide energy measurement. For example, a predeterminedsampling frequency of once every 24 cycles may be used. Such sampledcycles may be scaled and accumulated over a time period to provide anaccurate energy measurement (the energy may be expressed in watt-hours)for each output.

In some embodiments, the invention provides a PDU in which one moreoutputs of the PDU may be switched to provide or remove power to aparticular output. In some embodiments, a relay circuit has twotransistors, one that initially switches the relay and a separatetransistor for holding the relay with lower current draw. The secondtransistor is switched on a predetermined time following the initialrelay turn-on, and subsequently the first transistor is turned off, thusproviding a lower current flow through the relay coil. This arrangementuses less power than a switching transistor that holds the relay active.The resulting accumulated savings for numerous PDUs in a facility ornetwork can be significant in certain applications. Such reduced powerrequirements may also generate less heating related to power consumptionand may also enhance the lifetime of such components as compared toimplementations not having such reduced power requirements.

Some embodiments having switched output capabilities can provide outputswitching at or near zero voltage or current crossings of the AC cycleor at least likely below the peak voltage or current in the AC cycle. Insome embodiments, for example, the AC waveforms provided to an outputare sampled at or near the point of zero current or voltage crossingsand outputs may be switched at or near these zero crossings. Inembodiments in which relays switch outputs, at or near zero voltage orcurrent crossing can be less stressful on the relay and the relaypoints. This may result in increased component lifetime and reducedin-rush current into the component that receives power from therespective output to possibly also reduce stress on that component.

Some embodiments of the disclosure provide a modular construction of anoutlet assembly with options to provide switched outputs or non-switchedoutputs, and the ability to determine if lack of power at an outlet isthe result of loss of input power or a blown fuse.

Certain embodiments provide a multi-outlet power meter that monitors thevoltage and current delivered to one or more power outputs. A monitoringcircuit may include a voltage input having an analog-to-digitalconverter that samples the incoming voltage and provides a digitalrepresentation of the incoming voltage waveform. A current input foreach power output, connected to the meter, has an analog-to-digitalconverter that samples the current input and provides a digitalrepresentation of the incoming current waveform. The samples of each ofthe current and voltage waveform may be collected to provide powermetrics such as, for example, RMS voltage, RMS current, active power,apparent power, energy, power factor, and crest factor. This informationis transmitted from the meter toward network components that may thenreport the information to a remote user.

In some embodiments, the systems may reveal when one or more componentsof a power distribution unit, or components that receive power from thepower distribution unit, develop an abnormal current or voltagecharacteristic. Such identification may provide an indication of apotential failure of the component. In some embodiments, current andvoltage information are collected for a component and compared againstmodel or historical information. In the event that an anomalous event isdetected, a message may be transmitted indicating the same such that aninvestigation or corrective action may be taken.

In some embodiments, information provided by PDUs or CDUs may be used byan organization to take action such as, for example, corrective action,improving the efficiency of operations, providing power metrics forspecific components, providing more accurate billing for energy usage,or identifying equipment that may be a candidate for consolidatingoperations. In some embodiments, corrective action may be taken such asin the event that a CDU or PDU generates a warning that the current orvoltage waveforms of for example, a power supply have a significantdeviation from a historical or model waveform. In some embodiments, suchdeviations may indicate the power supply is malfunctioning andcorrective action can be taken. In some embodiments, power metrics maybe used to evaluate the operations of different items or groups ofequipment to identify areas where efficiency can be enhanced, forexample. Similarly, power metrics may be used to determine energy usage,and provide billing for separate pieces of equipment within a rack, forexample. Furthermore, power metrics also may be used to identifymultiple equipment, such as two servers, that are being used to servicean application where utilization of the equipment is such that theoperations may be consolidated and one or more pieces of equipment idledor placed in stand-by mode.

Individual equipment may be measured in order to determine various typesof information related to power efficiency metrics. In embodimentsdescribed herein, power-related information is determined for varioustypes of equipment, and this information provided to determine powerusage related to that equipment. In some embodiments, a PDU is providedthat senses and outputs the power used by various different components,including monitoring both the input power of the PDU and the poweroutput to various components powered by the PDU. For example, todetermine total computing equipment power (Power is (Volts×Amperes) orWatts) delivered by the PDU, the PDU could measure Watts for each inputcord to the PDU(s), or the input power at various subcomponents thatprovide power to one or more pieces of computing equipment. The sum ofall the power output to pieces of equipment measures the total computingequipment power delivered by the PDU and consumed by the computingequipment assuming all computing equipment assets are plugged into a PDUhaving the ability to measure power. In some embodiments, severalmetrics are calculated for each outlet in a PDU including voltage (trueRMS Voltage), current (true RMS Current), active power (Watts), apparentpower (VA), energy (Watt-hours), power factor (unitless), and crestfactor (unitless). Each of these metrics may be used alone, or incombination with other of the metrics, to provide information related tocomponents that are receiving power from the outputs of the PDU.

In other embodiments, an individual piece of computing equipmentefficiency may be determined using one or more of computed metrics, suchas MIPS/Watt. MIPS (million instructions per second) is a measure of thespeed of execution of a processor. Thus, performance efficiency for aparticular server, for example, may be measured or a cumulativeefficiency calculated for all equipment in a data center. In embodimentsthat provide such metrics, the metrics may be provided for each outletin the PDU, or for a subset of outlets in the PDU. The MIPS value of acomponent may be read over a network, for example, from the BIOS of thecomponent, and combined with power metrics to provide a measure ofperformance efficiency. The sum of all the ‘per outlet Watts’ on a PDUmay be used to measure the PDU's efficiency when compared to the inputcord power to the PDU. In some embodiments, an individual piece ofequipment may receive operating power from multiple power supplies. Insuch embodiments, the outlets that provide power to the piece ofequipment are grouped using an application external to the monitoringcircuit, with metrics from each outlet in the group summed to providethe corresponding metrics for the specific asset that is acquiring itpower from multiple PDUs, CDUs, or multiple power supplies. Otherembodiments provide the ability for an expense charge for the powerconsumed by each specific asset, and each outlet may record the amountof power used (Watt-hours) in the same manner as a utility meter.

With reference now to FIG. 1, a block diagram of an exemplary system ofan embodiment is now described. A power distribution unit (PDU) 20supplies power to one or more associated computing assets. As previouslynoted, the PDU 20, together with other devices connected to it, forexample environmental monitors, temperature and humidity sensors, fusemodules, or communications modules that may external to or containedwithin the PDU housing, may be collectively referred to as a CabinetDistribution Unit (CDU). The PDU is useable in a computer network 24,and may communicate over the computer network 24 with a network powermanager application 28. In cases where the PDU 20 is included in a CDU,communication with network power manager 28 is conducted through acommunications module within the CDU. The network power manager 28 mayreside in a workstation or other device that is used in the managementof a data center or other enterprise management, and issues networkcommands over a network communications connection.

The PDU 20 of this embodiment includes a power supply 32. It alsoincludes a network interface card (NIC) 34 that has application firmwareand hardware that allows the PDU 20 to communicate with other moduleswithin a CDU, and in this embodiment includes a power manager agentapplication 36. The PDU 20 includes a plurality of power outlets 40arranged within an intelligent power module (IPM) 44. The NIC 34, andpower manager agent 36 are connected to the computer network 24. Theintelligent power module 44 controls the application of power from theinput power to a corresponding power outlet among the power outlets 40,and is in communication with the power manager agent application 36 toprovide power and power cycling on-off for one or more of thecorresponding power outlets, which may be accomplished through one ormore relays 45 and associated relay driver 46. The IPM 44 receives inputpower, and provides power to one or more outlets 40 through the relays45. The IPM 44 may also provide power state sensing or load sensing withrespect to the corresponding power outlet in response to one or morecommands. The IPM 44 in this embodiment includes a microprocessor 48used to control the power applied to a corresponding power outlet. Themicroprocessor also is connected to a voltage sensing device 52 and acurrent sensing device 56 to sense the voltage and current atcorresponding individual power outlet(s). The microprocessor 48 usesthis information to determine the power supplied through an outlet, aswill be described in more detail below. The microprocessor 48 alsoreceives a power measurement from the input power supply 32 through aninput voltage sensing device and an input current sensing device 21internal to the PDU. In this embodiment, the IPM 44 also includes apower supply 58 used to provide DC operating power to components withinthe IPM 44. A display 23, for example a single- or multi-digit LEDdisplay, may be included to provide a visual indication of voltage,current or another power metric; the display is shown as communicatingwith the microprocessor 48 but could instead communicate directly withother elements such as the sensors 56 or 52. The display may instead bydriven by either the NIC 36 or the power supply 32.

The network power manager 28 of FIG. 1 communicates with the intelligentpower module 44 through the power manager agent 36. In this embodiment,the network power manager 28 may receive information from, and provideinstructions to power manager agent 36 which communicates with IPM 44.The network power manager 28 may also receive related power measurementsfrom the IPM 44 (through power manager agent 36) and report powerinformation related to the PDU 20, and related to one or more individualoutlets (and thus power information for individual assets powered by theoutlet) of the PDU 20.

FIG. 2 is an illustration of a CDU 65 that includes Intelligent PowerModules 200, along with a communications module 66 that providescommunications functions, an environmental monitor 68, and an inputpower cord 70 with associated plug 72. The Intelligent Power Modules 200each include eight outlets 202-216 that supply power to assets that maybe mounted into an equipment rack. Such equipment racks are well known,and often include several individual assets that are used in operationof a data center. The CDU 65, as illustrated in FIG. 2, is configured tobe vertically mounted in an equipment rack, commonly at the rear of therack adjacent to the rear side of electronic equipment mounted in therack. As is well known, numerous equipment racks may be included in adata center, and in various embodiments each asset in each equipmentrack may be monitored for power usage through one or more associatedIntelligent Power Modules 200. The visual display 23 (shown displayingthe numeral “57”) is disposed in the CDU 65 although in otherembodiments the display might be external to the CDU 65.

FIG. 3 is a block diagram of output power reporting components in anexemplary embodiment. In this embodiment, the PDU includes anIntelligent Power Module 200, also referred to as a power outlet module200, that includes eight power outlets, 202-216. Each outlet 202-216 isconnected to power lines L1 and L2 and then to power source 32. In thisembodiment, the power line L1 is connected to line power in the powersource 32, and the power line L2 is connected to neutral in the powersource 32. However, in other embodiments the lines L1 and L2 may beinterconnected to different phases of a polyphase power source. Eachoutlet 202-216 is also interconnected to a ground in the power source32, although this connection from the outlets 202-216 is not illustratedin FIG. 3. In this embodiment, each outlet 202-216 has an associatedtoroidal current sense transformer 202 a-216 a that senses currentflowing through the line L1 for each respective outlet 202-216. The lineL1 interconnected to each outlet 202-216 is wired through the respectivetoroid 202 a-216 a. The toroidal transformers 202 a-216 a each has acurrent reporting cable 202 b-216 b that provides instantaneous currentinformation related to the respective toroidal transformer 202 a-216 ato microcontroller 220. Current information may be determined usingother configurations, such as through the use of a shunt resistor, halleffect device, or other suitable current sensing device, as will bereadily recognized by one of skill in the art. Such other configurationsfor determining the current provided to an outlet may be used in otherembodiments The microcontroller 220 receives this current informationrelated to each respective outlet 202-216.

The power outlet module 200 also includes one or more line voltagesensors, each including a voltage dropping resistor network 224, and anopto-isolated operational amplifier 228 to provide instantaneous linevoltage information to the power source 32. The line voltage may also bedetermined through various other configurations. The line voltagesensor, for example, may include a voltage sense transformer thatprovides isolation and allows voltage to be determined based on thevoltage across the transformer and the turns ratio of the transformer.Other embodiments may not provide isolation, instead achieving isolationfrom high-voltages in other manners. The microcontroller uses thecurrent information related to each of the respective outlets 202-216,along with the line voltage to calculate the power metrics associatedwith each of the individual outlets 202-216. This information may becommunicated to other components through communications link 230through, for example, a communications bus. One or more voltage sensorsmay be provided for each power input if the PDU has more than one, whichwould be the case for example if polyphase power is provided. Or aseparate voltage sensor may be provided for each group of outlets oreven for each outlet individually. Using multiple sensors may provideimproved accuracy by avoiding the effect of any internal voltage dropsin the wiring between the power input to the PDU and the outlets.

In one embodiment, the power outlet module 200 includes eight outlets(202-216) each of NEMA 5-20R type, contained in a housing. It will beunderstood that this embodiment, and other embodiments described hereinas having NEMA 5-20R type outlets, are exemplary only and that any ofvarious other types of outlets alternatively can be used. For example,the “outlets” can be other NEMA types (e.g., NEMA 5-15R, NEMA 6-20R,NEMA 6-30R or NEMA 6-50R) or any of various IEC types (e.g., IEC C13 orIEC C19). It also will be understood that all “outlets” in a particularpower outlet module 200, or other module-outlet described herein, neednot be identical. It also will be understood that the “outlets” are notlimited to three-prong receptacles; alternatively, one or more of the“outlets” can be configured for two or more than three prongs in themating male connector. It also will be understood that the “outlets” arenot limited to having female prong receptacles. In any “outlet,” one ormore of the “prong receptacles” can be male instead of female connectionelements, as conditions or needs indicate. In general, as used herein,female and male “prong receptacles” are termed “power-connectionelements”. Furthermore, the principles described herein also areapplicable to devices that may be hard-wired into an outlet module.While outlet module 200 of this embodiment includes eight outlets, itwill be understood that this is but one example and that an outletmodule may include a different number of outlets.

The housing for an outlet module may be any suitable housing for such adevice, as is known to one of skill in the art, and may be assembledwith other modules in a CDU. Such a housing generally includes a frontportion and a rear portion, the front portion is substantially planar,and the rear portion is substantially planar and parallel to the frontportion. The housing also includes longitudinally extending sideportions and transverse end portions. The front portion, rear portion,side portions, and end portions are generally orthogonal to each otherin a generally rectangular or box-type configuration. The housing can bemade of any suitable, typically rigid, material, including, for example,a rigid polymeric (“plastic”) material. In at least certain embodiments,the front and rear portions are made from an electrically insulativematerial, whereas in other embodiments conducting materials are used forsafe ground bonding. The side portions and the end portions may beintegrally formed, optionally along with the front portion or the rearportion. Furthermore, while the outlet module described in thisembodiment includes a housing, other embodiments may include an outletmodule that does not include a housing. For example, an outlet modulemay include a number of outlets coupled together with no exteriorhousing that may then be installed into another piece of equipment.

Each outlet 202-216 is interconnected to the power source 32 through anyof a number of well known connection schemes, such as spade, lug, plugconnectors, screw connectors, or other suitable type of connector.Furthermore, if desired, one or more of these electrical connectors canbe located inside the housing or outside the housing, in embodimentswhere the power outlet module includes a housing.

The microcontroller 220, in this embodiment, receives currentinformation for each outlet 202-216, along with voltage information andcalculates various power-related metrics for each outlet, with thisinformation reported through the communications link 230. For example,the power per outlet is determined by multiplying the instantaneousvoltage by the instantaneous current for a particular outlet, andintegrating this product over time to give energy used (kilowatt hours;etc.) Examples of several metrics will be discussed in more detailbelow.

With reference now to FIGS. 4-8, schematic diagrams of an exemplaryembodiment are now discussed. In this embodiment, various differentcomponents of an outlet module may be assembled onto separate circuitboards that are then assembled into an Intelligent Power Module. In sucha manner, component boards may be assembled to include features that areordered by a particular customer or user of a PDU in which the outletmodule will be used. Furthermore, a user or customer may desire some,but not all, of the outlets in a PDU to have the capability of reportingpower usage related to individual outlets, and thus different outletmodules, or subsets of outlets in a outlet module, may be assembled withthe additional component boards to provide such capability. Similarly,in the embodiment of FIGS. 4-8, each outlet in the outlet module may beindividually switched on or off through a remote power manager. However,other embodiments do not provide such switching capability, and thecomponents described with respect to switching outlets would thereforenot be included in such embodiments, replaced instead with simplepass-through components.

In this embodiment, an outlet module includes eight (8) individualoutlets organized into logical groups of four outlets each. FIG. 4provides a partial schematic illustration of an outlet circuit 500 forsuch an embodiment. In this embodiment, eight outlets 502-516 areassembled to be included in an outlet module (in FIG. 4 only theconnection points, not the actual outlets, are shown). In thisembodiment, outlet 502 and 516 are IEC-C19 type connectors, and outlets504-514 are each IEC-C 13 type connectors, although it will be readilyrecognized that outlets may be any suitable outlet type as required fora particular application. The outlet circuit 500 includes a ground input520 that is electrically connected to a ground connection in eachrespective outlet 502-516. A neutral line may be electrically connectedto each outlet 502-508 through a neutral input 524 that is provided forthe four outlets 502-508, with a neutral line electrically connected toeach outlet 510-516 through a second neutral input 528. Alternatively,if all eight outlets 502-516 are to be connected to a single powersource, the neutral line for each set of four outlets may be connectedthrough a hard-wire jumper connecting the two connection points 532;this allows a neutral connection to either the connection point 536 orthe connection point 540 to electrically connect the neutral for eachoutlet 502-516. As will be readily understood, a line voltage may beprovided in place of a neutral connection in applications requiringhigher voltages for the outlets 502-516.

With continuing reference to FIG. 4, this embodiment provides a visualindicator at each outlet 502-516 that voltage is present at the outlet502-516. The visual indicator is provided through a LED 544 that isinterconnected between line power and neutral for each outlet 502-516.Line power for each outlet 502-516, in this embodiment, is providedthrough line inputs 548-562. Each line input 548-562 may be connectedthrough a switch to line power from a power source, as will be describedin more detail below. In such a manner, when a respective switch isconfigured to supply power to an outlet 502-516, the LED 544 associatedwith the outlet 502-516 will illuminate, thus providing a true visualindication that voltage is being provided to a particular outlet502-516. The LED 544, in this embodiment, is electrically connectedbetween the line input and neutral through current limiting resistors570 and diode 566. In other embodiments, such a visual indicator may notbe desired, and in such embodiments the components related to the visualindicator may be omitted. As mentioned, line power is provided throughseparate line inputs 548-562 for each respective outlet 502-516. In someembodiments, the line inputs 548-562 are electrically connected toswitches to provide switched electrical outputs 502-516, and in otherembodiments some or all of the line inputs 548-562 may be connected inan unswitched configuration to a line power input to provide unswitchedoutputs.

As mentioned, in some embodiments switched outputs are provided. Withreference now to FIG. 5, provided in this embodiment is a relay circuit600. The relay circuit 600 may be provided on a separate printed circuitboard that is configured to couple with the outlet circuit 500. In sucha manner, if switched outlets are required for an outlet module, therelay circuit may be assembled with the outlet module to provide suchfunctionality. When switched outputs are not provided, this circuitboard may be replaced with a simple pass-through circuit board havingthe same connections to other circuit boards, simplifying assembly andmanufacturing of such power outlet modules. The relay circuit 600includes relays 602-616 that provide line power to each outlet 502-516,respectively. The output of each relay 602-616 is provided to line poweroutputs 648-662 that, when coupled to outlet circuit 500, are connectedto line inputs 548-562, respectively. When all eight outlets 502-516 areto receive power from one line power input, line power jumper 670 isinstalled and line power is provided to the relay circuit 600 througheither power inputs 672 and 674. When a line power input is provided foreach set of four outlets 502-518, and 510-516, line power jumper 670 isremoved and line power is provided through power inputs 672 and 674.

Each relay 602-616 is connected to a relay driver circuit 678-692,respectively, that provide signals to switch the relays 602-616. Therelay driver circuits 678-692 are electrically connected through aconnection 696 to a microcontroller 904 (see FIG. 8) via a decodinglatch 908. In this embodiment, relay driver circuits 678-692 eachinclude a switching transistor 698 and a holding transistor 699. Whenthe relay control circuit provides voltage to switch a particular relaydriver circuit 678-692, the voltage is applied directly to the holdingtransistor 699 and switching transistor 698 through a capacitor 700 anda resistor 702. In this manner, upon the application of voltage to therelay circuits, both the switching transistor 698 and the holdingtransistor 699 receive voltage and act to switch the respective relay602-616 and connect line power to the respective outlet receptacle.After a short time period, the capacitor 700 charges and reduces currentflow through resistor 702 such that the voltage at the switchingtransistor 698 drops and the switching transistor 698 switches off. Theholding transistor 699 continues to provide adequate voltage to hold therespective relay 602-616 closed with reduced current through currentlimiting resistors 703.

In such a manner, the power required to hold the relays 602-616 isreduced as compared to the power required to initially switch the relays620-616 from open to closed. In one embodiment, the holding transistorrequires about 75% of the power to maintain the relays 602-616 closedthan would be present if a single transistor were used to both switchand hold. In embodiments where numerous switched outlets are present ina facility, such power savings can be significant in operating powerreduction for the associated CDUs, which in turn reduces heating, allowsfor increased component density on a circuit board or within a housing,and also increases the lifetime of components. Other embodiments,however, may include different switching components as will be readilyapparent to one of skill in the art.

With reference now to FIG. 6, current sensing is described for thisembodiment. A current sensing circuit 710, in this embodiment, isincluded as a separate printed circuit board that can be assembled intoa power outlet module when it is desired to have the capability toprovide current information related to each individual outlet in anoutlet module. Such a circuit board may be used in conjunction withother circuit boards, such as the relay circuit 600 of FIG. 5. Such aconfiguration is illustrated in FIG. 14, in which the circuitry of FIG.4 is contained on the bottom printed circuit board 750, the circuitry ofFIGS. 6-8 is contained on the middle circuit board 754, and thecircuitry of FIG. 5 is contained on the upper circuit board 758. Theelectrical connections of each of the circuit boards may be designedsuch that the boards may be assembled with related inputs/outputs andconnections that are aligned so as to provide for efficient modularassembly of power outlet modules that incorporate some or all of thefeatures described herein through the addition of one or more relatedprinted circuit boards. In some embodiments the circuits shown in FIGS.5, 6 and 7 are all contained on one printed circuit board.

As illustrated in FIG. 6, current transformers (CTs) connected toconnection points 712-726 are provided that sense current flowing in anassociated conductor that is routed through the individual currenttransformers. The CTs in this embodiment are zero-phase toroidalinductors that each have two output lines, the output proportional tothe magnitude of the current that is flowing through the conductorassociated with the current CTs. In this embodiment, the line powerconductor for each outlet 502-516 is routed through a corresponding CT.The respective CT 712-726 outputs a signal that corresponds to themagnitude of the current which, in this embodiment, is output on twooutput leads across a burden resistor 730. This configuration providesthe ability to sense output currents up to 16 amperes with a maximumcrest factor of 2.5, although it will be readily apparent to one ofskill in the art that other configurations are possible.

In the embodiment of FIG. 6, each CT is connected to a related passivetwo-pole anti-aliasing filter 732, 734 to provide current sense outputs712 a, 712 b through 726 a, 726 b for each outlet. The current senseoutputs 712 a, 712 b-726 a, 726 b are provided as differential input toa microcontroller differential analog-to-digital input for use indetermining the power metrics related to a particular outlet. Alsoprovided to the power sensor is information related to the line voltagethat is present on each outlet so as to provide voltage and currentinformation for use in determining power metrics. In this embodiment, aswill be described in more detail below, the power sensor is amicrocontroller that includes an analog-to-digital converter with inputsfor the current sense outputs 712 a, 712 b through 726 a, 726 b, as wellas voltage sense inputs for line voltage.

Line voltage measurements are provided, in this embodiment, through avoltage sensor circuit 800 that is illustrated in FIG. 7. The voltagesensor circuit 800 includes a voltage divider consisting of a resistor804 and a resistor 805 connected in series across the line power betweena first end 808 and a neutral input at a second end 812. Positive andnegative voltage inputs to an opto-isolated amplifier 816 are connectedacross the resistor 805. Similarly as described above, other voltagesensing circuits may be used, such as a voltage sense transformer may beused instead of a voltage dropping resistor network, for example. Also,in some embodiments voltage sensing may be provided that is notopto-isolated with any required isolation provided by other well knownmethods. The output of the opto-isolated amplifier 816 is provided as avoltage sense signal at an output 820 through a passive two-poleanti-aliasing filter consisting of resistors 806 and 807 and capacitors809 and 810.

An opto-coupler 824 is connected to the line input through a diode 814and a resistor 815 and provides a frequency sense signal at an output826 to indicate that AC line voltage is present at the Intelligent PowerModule and also provides an approximately 50% duty cycle output that isbased on the line frequency of the input power. Thus, for each AC cycleof the input power, the frequency sense signal will have a logical highsignal for approximately one half of the AC cycle. The leading ortrailing edge provided by the frequency sense signal provides anaccurate measurement of the frequency of the input voltage frequencythat may be used by a processing circuit to synchronize power metrics toan AC cycle.

In embodiments where all of the outlets of an outlet module are poweredby a single power source, a single voltage sensor circuit 800 is used,and in embodiments where different outlets in the outlet module aresupplied power from different power sources, a second voltage sensorcircuit is provided for the second power input to the outlet module. Asdiscussed above, this embodiment may be implemented using printedcircuit boards that provide circuitry for various features described. Inthis embodiment, the voltage sensor circuit(s) are provided on the sameprinted circuit board as the current sensor circuit 710, although itwill be readily recognized that other configurations may be implemented.

Referring now to FIG. 8, a power sensor and control circuit 900 isdescribed for an embodiment. The power sensor and control circuit 900,in this embodiment, is included on the same printed circuit board as thecurrent and voltage sensor circuits 700. 800, although otherimplementations will be readily recognized. The power sensor and controlcircuit 900 includes a microcontroller 904 that receives all of thecurrent sense signals 712 a, 712 b through 726 a, 726 b, and receivesvoltage sense signal(s) 820. These signals are received and processed todetermine the power metrics related to each outlet 502-516 in the outletmodule. The microcontroller 904 is interconnected to an addressablelatch 908 that provides control signals to the relay drivers 678-692 andrelays 602-616, if present. The microcontroller 904 also includescommunications connections 912 that may be coupled to a communicationsbus to receive and transmit data from/to the bus. In this embodiment,the microcontroller 904 has 16 current input channels, two per outlet,which are electrically connected to the current sense outputs 712 a, 712b through 726 a, 726 b, and two voltage input channels which areelectrically connected to voltage sense output(s) 820. Themicrocontroller includes ADC inputs that digitize the current andvoltage sense signals. Relative to the current sense signals, the ADCincludes a differential ADC input based on the two inputs from thecurrent sensor associated with each outlet.

In this embodiment, the microcontroller 904 filters the current andvoltage sense signals to reduce high-frequency noise that may bepresent. The digitized current sense signals are scaled for 16 Amps witha 2.5 crest factor, in this embodiment. The voltage sense signals(s) arereceived on voltage input channels. In embodiments having differentpower sources for some outlets, one voltage input channel per outletgroup is provided. The voltage input channels are provided to asingle-ended ADC input and a digitized output scaled for +/−390 voltpeaks. The frequency sense signals for each power source are alsoprovided to the microcontroller. The frequency sense signal(s), in someembodiments, is (are) used for frequency determination and timing ofcycle sampling to provide accurate correlation of inputs to a particularAC cycle. The timing, in an embodiment, is auto-adjusted every second tocompensate for inaccuracies, such as temperature drift, in the internalclock of microcontroller 904.

Use of the frequency sense signal 826 provides for accurate timing inthe microcontroller 904 without the use of an external oscillator as anaccurate time base. The ability to measure the frequency sense signal826 provides enhanced accuracy for timing used in calculatingpower-related metrics for each outlet. In this exemplary embodiment, twosignal types are digitized by an ADC within the microcontroller, thevoltage and current signals. Synchronizing the sampling of both thevoltage and current waveforms, utilizing frequency sense signal 826,provides for enhanced accuracy in the power-related measurements. It iswell known that internal clocks in microcontrollers such asmicrocontroller 904 have some variability, such as plus or minus twopercent. Such internal clocks are typically subject to frequency shiftwith changing temperature, and also have variability between differentmicrocontrollers. In this embodiment, the frequency sense input allowsfor real-time compensation of the microcontroller internal clockvariance to insure accurate sampling of the AC voltage and currentwaveforms. The voltage and current sense inputs on the microcontroller904 are sampled nearly simultaneously 120 times per any AC cycle. Thenumber of samples per cycle, 120 in this example, provides sampling offrequency content up to the 14th harmonic of a 50 or 60 hertz powerinput, allowing for measurement of real energy at harmonics present in anon-perfect sinusoid. The ADC, in an embodiment, within themicrocontroller is a 10-bit ADC hardware, with four times over-samplingto provide an effective 11-bit ADC.

The computation of several power metrics will now be described, for anexemplary embodiment. In this embodiment, discrete samples are taken forone current and voltage channel for an AC cycle, which produces adigital measurement at each sample interval. After the samples are takenover one cycle, calculations are performed by the microcontroller, thesecalculations consume about one-and-a-half AC cycles in this embodiment.After the calculations are performed, the next channel is sampledbeginning at the start of the next AC cycle. Thus, in this embodiment,there are three cycles dedicated to the first channel, the next threecycles dedicated to the second channel, and so on. Accordingly, in thisembodiment with eight outputs monitored, each channel is sampled onceevery 24 AC cycles.

Also, voltage and current inputs are calibrated and provided to themicrocontroller 904 in some embodiments. The current inputs, in anembodiment, are scaled to 16 amps at 2.5 crest factor and with thevoltage input(s) scaled for 390 volts. Variances in the resistors andtoroids, in an embodiment, is accounted for through calibration of theinput channels. In one embodiment, the voltage and the current arecalibrated based on active power and apparent power for each channel,although calibration based on other metrics may be used, such ascalibrating the voltage and current individually. In embodiments thatcalibrate voltage and current individually, any errors that are inopposite directions will tend to cancel, and any errors in the samedirection will be multiplied, when doing a power calculation. Inembodiments that calibrate based on active and apparent power, themultiplied error may be reduced. The microcontroller 904, in thisembodiment, also provides for calibrations to account for system phaseerror and provide near-zero to near-full-span voltage and near-zero tonear-full-span current digitization.

With reference now to FIG. 9, a block diagram illustration of amicrocontroller 904 is provided for an exemplary embodiment. Themicrocontroller 904, as mentioned above, includes an analog-to-digitalconverter 906 that receives an input from the current sensors and thevoltage sensors. Samples from the ADC 906 are provided to processinglogic 908. A memory 910 is interconnected to the processing logic 908and may be used to store information related to power metrics andsampled current and voltage information, as well as any programming usedby the processing logic. An internal clock 912 provides an internal timebase, and as discussed above the processing logic 908 also receives afrequency sense signal that allows accurate synchronization with an ACcycle. The microcontroller 904 also includes a relay control 914 and acommunications interface 916. The communications interface may be usedto receive and transmit information to and from a communications bus,such as power metrics computed by the processing logic, control commandsto actuate different relays through the relay control 914, etc.

With reference now to FIG. 10, the operational steps of amicrocontroller for determining power metric related information aredescribed for an exemplary embodiment. In this embodiment, the ADC 906is a 10 bit ADC, with single-ended channels for voltage sense inputs anddifferential channels for current sense inputs. As mentioned above, 120samples of voltage and current are taken for each cycle in anembodiment. Each of those samples, 120 over the AC cycle, are takennearly simultaneously for both the current and voltage. In anembodiment, near simultaneity is achieved by successive ADC sampleswhich are only microseconds apart, for a relatively small error effecton overall calculations.

The operational steps shown in FIG. 10 begin with sampling the currentand voltage waveforms from the sensors for one input power cycle andstoring the resulting digitized data (1001). Next, raw RMS values of thevoltage and current are calculated (1003) and these raw values arescaled by a predetermined calibration value to obtain true RMS valueswhich are then stored (1005). Average active power consumed during onecycle (1007), average apparent power (1009), power factor (1011), inputcurrent crest factor (1013), phase angle between voltage and current(1015) and energy usage (1017) are also calculated and stored.

FIG. 11 shows an embodiment of the invention in which voltage andcurrent data are compared with historical data or with predeterminedmodel data. If the voltage and current data are to be compared withhistorical data, the historical waveform data are retrieved from storage(1101). If a predetermined ideal model waveform is to be used, datarepresenting that waveform are retrieved from storage (1103). Then theretrieved data are compared with the stored samples (1105) to determineif there are deviations (1107). If deviations exceed a predeterminedthreshold, for example if the stored samples representing the measuredvoltage and current show a waveform distorted beyond a predeterminedlimit, an unexpected power factor, too much or too little power beingconsumed, or the like, the deviation is reported (1109). A record of thecomparison may be stored (1111). The process may be repeated asindicated by a line 1113 returning back to step 1105.

FIG. 12 shows a summation over time of energy used. Power line frequencyis calculated (1201), and the power line signal is thereupon used as atiming signal to accurately measure the time during which energy isbeing consumed. The power consumed in a defined number of input powercycles, for example 24 cycles in some of the embodiments discussedherein, is calculated (1203) and is added to a record of power alreadyconsumed (1205) to maintain a running total of the power consumed.

FIG. 13 shows further use of timing information derived from the inputpower signal. Negative edges of the input voltage cycle are detected(1301) and the internal clock pulses between successive negative edgesare counted (1303) to obtain the number of internal clock pulses pervoltage cycle. This number is adjusted for system latencies and anyknown system errors to obtain a reload value (1305). The reload value isused as a timing basis for sampling the voltage and current as measuredby the sensors (1307). When a predetermined interval has been reached,the number of clock pulses per cycle can be remeasured (1309).

The procedures illustrated in FIGS. 10 through 13 will now be describedin more detail. Each voltage and current sample is stored in memory 910as an integer value. For each set of current and voltage samples, theprocessing logic calculates the true RMS voltage and current in severalsteps. First, each data point in the 120 samples is summed together andthen divided by 120 to get the mean of the samples, as a floating pointvalue. Then, for each sample, the processing logic calculates thedifference of that sample from the mean as floating point values. Eachdifference from the mean is squared, and the sum of the square of everypoint's difference from the mean is calculated. This total sum isdivided by 120. The raw RMS value is then determined as the square rootof the resulting quotient. This number is scaled by the calibrated scalefactor to produce a calibrated value, referred to as a true RMS value,which is stored in memory 910 for both the set of the current datapoints and the set of voltage data points. The result is RMS current andthe RMS voltage values. In this manner, an AC RMS value is generatedthat removes any DC offset present from the sensing circuitry or thesignal itself.

In one embodiment, the samples of voltage and current in a waveform arecompared against a model waveform or a historical waveform for thatparticular channel, and any significant deviations from the comparisonmay be flagged as anomalous indicating that there has been a changerelated to the associated component. Such a change may indicate thecomponent may not be operating properly, may be about to fail, or mayhave had a failure. For example, waveforms of the current drawn by adevice and the voltage drawn by the device, when compared to historicalor reference waveforms, may indicate a fault or other condition thatshould be investigated. For example, a switched-mode power supplylocated within a server that receives power from a PDU may be drawingpower in a manner that indicates an imminent failure. Embodimentsdescribed herein provide the ability to assess the health of such powersupplies in an installed base of power supplies in data center equipmentracks without requiring any modification of the power supplies.

In some embodiments, currently sampled waveform information is onlymaintained in memory long enough to be utilized to generate and reportthe noted power metrics. Other waveforms, however, may be maintained inmemory for comparison, such as in the form of or representative of oneor more sample or reference waveforms or portions of one or morewaveforms. In addition, the waveform information might be maintained inmemory longer or otherwise stored for later use in, e.g., providing abasis for comparison. For example, when a system is initially set up andtested, the waveform may be stored and used for later comparison.

Referring again to FIG. 10, power for each cycle is determined by first,for each of the 120 data points for current and voltage, calculatingproducts of each respective sample. These 120 products make up thewaveform of the wattage that may be compared to model or historicalwaveforms to identify any potential problems related to the componentthat is receiving power from the associated outlet. The sum of theproducts of each current and voltage data point is then divided by 120to get the average power, referred to as active power. It is noted that,in this embodiment, zero-phase toroidal current transformer are used andthe voltage and the current samples are digitized approximatelysimultaneously, and thus the phase angle created by loads is inherent inthis measurement. This phase angle may be determined as the inversecosine of the power factor, as will be described in more detail below.

Also calculated is apparent power, which is the product of the RMScurrent and the RMS voltage calculated earlier, having units ofvolt-amps or VA. Power factor, the ratio of the active power to theapparent power, is calculated, which directly relates to the phase angledifference between the current and voltage. Power factor is calculatedby taking the active power calculated from all the data points dividedby the apparent power, which was the product of the RMS current andvoltage. The next item calculated in this embodiment is current crestfactor. The current crest factor is the ratio of the peak of the currentwaveform to the RMS of the current waveform.

Finally, energy is calculated. As mentioned above, embodiments areprovided in which the microcontroller does not receive a time base froman external oscillator. The timing for such embodiments is based oncycles of the incoming AC waveform. As is well known, frequency ofincoming AC power is generally 50 Hz or 60 Hz, depending upon location.Furthermore, most, if not all, industrialized nations have electricalgeneration and distribution systems that provide a relatively stablefrequency of incoming AC power. The stability of incoming AC frequencymay be used to provide a relatively accurate timing mechanism forstarting and stopping ADC conversions. As described above, oneembodiment samples eight channels over the course of 24 AC cycles. Therelative accuracy of the incoming AC signal as a time base providesknowledge that there is an accurate measuring every 24th cycle for eachchannel with very little drift.

In an embodiment, the input power signal is sampled to determine if theinput power is 50 Hz or 60 Hz. At 60 hertz there are 216,000 cycles inan hour, and at 50 hertz there are 180,000 cycles in an hour. With thisinformation, and the measurement of one current channel every 24 cycles,energy may be calculated by multiplying the active power times 24,representing the total of all 24 cycles between measurements on achannel, and dividing by either 180,000 (at 50 hertz) or 216,000 (at 60hertz). This provides a representation for power consumed by the channelduring the 24 cycles. This energy computation is added to an energyaccumulator associated with each channel. Each time the power for achannel is computed, the wattage use for the represented 24 cycles isadded to the accumulator. In one embodiment, to reduce floating-pointsignificance and rounding errors, when the accumulator (a floating pointdata type in memory) exceeds one, the accumulator is decremented and adouble word integer associated with the channel is incremented toprovide a number representing whole watt hours that have been measuredfor the channel. All of the values stored in memory may be reportedthrough the communication interface to power managers or otherapplications that may then use this information to provide a number ofdifferent power-related metrics for components that receive operatingpower from the PDU.

As discussed above, relatively accurate timing is achieved inembodiments with a relatively high variability internal microcontrollerclock though adjustments that compensate for inaccuracies in theinternal clock. The compensation is achieved, in an embodiment, throughproviding the frequency sense input into an external interrupt pin onthe microcontroller. The frequency sense signal, as discussed above withrespect to the embodiment of FIG. 7, may be generated from a photo-opticdiode 824. As the voltage rises on the input power, the LED of thephoto-optic diode turns on, and the LED will turn off slightly above thezero crossing of the input waveform, regardless of the duty cycle. As aresult, every second edge of the frequency-sense signal is the frequencyof the line input. The microcontroller, in this embodiment, isprogrammed to identify a positive edge of the frequency sense signal.

Once a positive edge is identified, then the first negative edge isidentified. The interrupt within interrupt service routines for theexternal interrupt pin in the microcontroller is set to high priority tohave relatively few, if any, interruptions from any other softwareinterrupt service routines. When the first negative edge is detected,the microcontroller starts running a counter that counts every 12 clocksof the internal clock 912. In one embodiment, the internal clock 912 isnominally a 24.5 megahertz internal clock plus or minus 2%. The timerruns until the next negative edge is detected. Thus, regardless of thetiming of the internal clock 912, a number of system clocks isdetermined that represents the span of time, from the microcontroller'sview, of a single AC cycle. This number is converted into entire systemclocks for an AC cycle by multiplying by 12, and then divided that bythe number of samples collected within a single AC cycle (120 in thisembodiment). Thus, a number of clocks is calculated that represents thetime span for each sample of an AC cycle. This time is adjusted forexpected interrupt latencies in the microprocessor, due to known entryand exit times in the interrupt service routines, etc., to generate anumber of system clocks that represents the AC cycle. This value becomesa reload value for the timer that starts off each ADC conversion.

Thus, the timer becomes a time base for the digitizer of the ADC, andcontinues to be the time base for cycles when digitizing is notperformed. Errors in the time base may accumulate over time. In oneembodiment, errors are reduced by periodically re-measuring the numberof system clocks in an AC cycle, such as once every five seconds. Suchre-measuring provides adjustment to account for the actual speed of theinternal clock, which is susceptible to temperature change, and alsosynchronizes the timer to a zero crossing of the voltage waveform. Suchtiming and synchronization of timers to an AC cycle provides relativelyaccurate power metrics. For example, if an external crystal time basewere used, which is also susceptible to temperature change andvariability of the incoming AC signal, errors can be introduced inbetween the timing of AC cycles and also synchronization to AC cycles.In the embodiments described here, the timer is re-synced to providegreater confidence that the samples used for RMS calculations are withinthe actual AC cycle. If RMS calculations are based on samples that beginafter the cycle begins, or that end after the end of the cycle, errorcan be introduced to report either less or more energy than is beingintegrated. By re-syncing, sampling is more likely to be within a cycleand not outside the cycle, and thereby improves accuracy.

As mentioned above, to determine energy, an accurate measure of time isneeded to provide, for example, a watt-hours number. The abovedescription relies on the assumption of 50 or 60 hertz input signalbeing accurate. In some embodiments, the time as measured in themicrocontroller is compared to time provided by a network controller toverify or adjust energy calculations. In one embodiment, the number ofcycles counted in a timeframe of an hour is provided to a network cardand compared to an actual real time clock view of an hour. In the eventof any significant deviation, the network card may add a simplecorrection scale in for that. For example, if the microcontroller countsup number of clock cycles in an hour and reports to the network card,which measures 59 minutes, a simple adjustment may be made to the energyvalue.

In another embodiment, the timing of the AC cycles provides anindication related to when the incoming power waveform is at azero-crossing. In this embodiment, the switching on and off of therelays (such as in FIG. 5) is performed around the zero-crossings on thevoltage AC waveform, or at least at a point less than the peak value ofthe waveform. Such switching acts to reduce noise from the relays whenswitching, and may also extend the life of the relays. Reduced noiseresults, in part, because switching at a zero-crossing results inrelatively low, or no, voltage potential at the physical points withinthe relay, thereby reducing noise when the relay is switched.Furthermore, the point life of such relays may be extended due to lowerstress than would be present when switching occurs with a relativelyhigh voltage present at the relay. A further advantage of switching ator near zero-crossings is a reduction in the in-rush currentsexperienced by a piece of equipment. For example, if the points on arelay are closed as the top of the sine wave, the in-rush current wouldbe significantly higher than present if switching is performed at ornear a zero-crossing. In this manner, the entire chain of current pathis also less stressed.

While described above with respect to a CDU, it will be understood thatthe power measurement circuitry and portions thereof have manyapplications beyond the exemplary embodiments described above. Forexample, a low-cost power metering circuit such as described may beincorporated into other equipment to provide information related topower parameters for the particular equipment. A server may, forexample, include a power circuit as described to provide power-relatedinformation that may be used to assist in managing efficiency of theserver by, for example, identifying that a server is not operatingefficiently and that the load being serviced by the server may be atarget to be moved to a different server. Similarly, it has been desiredto have a switched-mode power supply that provides power-relatedinformation, but there is a strong desire to maintain as low a cost forthese power supplies as possible. A single-chip solution without anexternal oscillator time base as described herein may provide a low-costsolution for incorporation into such power supplies. Further, such powermetering may be incorporated into residential, commercial and/ormultiple-unit power meters to provide power-related information forbilling purposes.

With reference again to FIG. 7, as mentioned above an outlet module mayinclude power outputs that are connected to separate line inputs. Insuch cases, separate voltage sensor circuits 800 are used for each setof outlets. Separate voltage sense circuits for each branch of outletsmay be desired for a number of reasons, such as separate branchesprotected by different fuses or circuit breakers, and one branch mayhave a fuse blown or the circuit breaker tripped and it could be offwhile power is still being supplied to the other outlets on the secondbranch. Also, those two branches may be operated at different voltages,for instance in a three-phase 208 volt wye system where both 208 volt AC(line to line) and 120 volt AC (line to neutral) may be present andrequired to be on separate branches. Two volt sense circuits 800 allowthe two different voltage values in that split branch configuration tobe measured and used in power metric calculations. Also, the on-sensemay be used to detect an absence of voltage that may result from manydifferent sources, one being a fuse or circuit breaker that has faulted.In cases where an on-sense signal is provided at the power cord input,it can be determined whether the line has failed or a fuse has blown.

As discussed above, the microcontroller 904 is interconnected to acommunications bus (such as an I2C bus or SMBus). The microcontroller904 reports over the bus, for each outlet/channel: (a) Voltage RMS(Vrms)—the pseudo-running-average of the eight most-recent Vrms valuesreported to a tenth volt; (b) Current RMS (Irms)—thepseudo-running-average of the eight most-recent Irms values reported toa hundredth Ampere; (c) Apparent Power (VA)—the pseudo-running-averageof the eight most-recent VA values reported to in volt-amps; (d) ActivePower (W)—the pseudo-running-average of the eight most-recent activepower values reported in watts; (e) Power Factor (pF)—thepseudo-running-average of the eight most-recent pF values reported to atenth; and (f) crest factor. This data may be received by an externalsystem that collects the outlet information for which the data isprovided, and used to determine metrics or provide information such asdescribed above.

FIG. 15 is an illustration of a single circuit board configuration of anembodiment. In this embodiment, the components described above withrespect to the three circuit boards as illustrated in FIGS. 4-8 areprovided on a single circuit board. In this embodiment, power outlets950 are provided that have a neutral line and a ground that are providedby a bus bar (not shown). The line power is provided to outlets 950through a line connection 954 that is routed through a relay 958 and anassociated current transformer 962. The relays 958 and currenttransformers 962 are interconnected to control and monitoring circuitrysuch as illustrated in FIGS. 4-8. In this embodiment, the printedcircuit board 966 is mounted at a 90 degree angle relative to the planeof the outlets 950. In this manner, the additional surface area requiredby the circuit board 966 is provided in a plane that is generallyperpendicular to the plane of the outlets 950, rather than in a parallelplane as illustrated in the embodiment of FIG. 14. By configuring thecircuit board 966 perpendicular to the plane of the outlets 950, thisadditional surface area can be accommodated simply be making the PDUhousing somewhat deeper, with the width of the housing remainingsubstantially the same as the embodiment of FIG. 14. Using a singleprinted circuit board 966 allows a reduced manufacturing cost andprovides efficiencies in manufacturing due to reduced assembly stepsrelative to embodiments with more than one printed circuit board.

Those of skill will appreciate that the various illustrative logicalblocks, modules, circuits, and algorithm steps described in connectionwith the embodiments disclosed herein may be implemented as electronichardware, computer software, firmware, or combinations thereof. Toclearly illustrate this interchangeability, various illustrativecomponents, blocks, modules, circuits, and steps have been describedabove generally in terms of their functionality. Whether suchfunctionality is implemented as hardware, software, and/or firmwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

For a hardware implementation, the processing units may be implementedwithin one or more application specific integrated circuits (ASICs),digital signal processors (DSPs), digital signal processing devices(DSPDs), programmable logic devices (PLDs), field programmable gatearrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, or a combination thereof. For afirmware and/or software implementation, the methodologies may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein.

A power management system embodying aspects of the invention isillustrated in FIG. 16. A first equipment cabinet 2401 houses components2403, 2405, 2407, 2409, 2411 and 2413. Also in the cabinet are a firstCDU 2415 and a second CDU 2417. The CDUs are shown outside of, andlarger than, the cabinet for convenience. Each CDU is similar to the CDUdepicted in FIG. 2. The component 2403 is shown both installed within,and outside of, the cabinet. The component 2403 draws power from bothCDUs as indicated by a cord 2419 connecting the component 2403 to anoutlet in the first CDU 2415 and a cord 2421 connecting the component2403 to an outlet in the second CDU 2417. Others of the components maybe connected to one or both of the CDUs as desired.

Similarly, a second equipment cabinet 2433 houses various components andone or more CDUs that provide power to these components. The system mayinclude other equipment cabinets having more or fewer components or CDUsthan depicted in the drawing.

The CDUs in the various cabinets communicate, for example through anEthernet pipeline 2425 or through the Internet or some other suitablemedium, with a server 2427. The server 2427 includes a database 2429which may be stored in a memory, or on a magnetic disk or other medium.The database 2429 may be located in one place or distributed as desired.In some embodiments the server 2429 communicates with another systemsuch as a Building Management System 2431.

As discussed previously, various electrical parameters respecting one ormore of the outlets may be measured and used in managing powerthroughout the system. Current flow through each outlet, voltage presentat the outlets, power factor, real and apparent power flowing througheach outlet, phase angle between voltage and current at each outlet,accumulated energy for each outlet, power line frequency, and the likemay all be measured and the measurements communicated to the server forpresentation to a user or for preparing reports, generating messages,providing trends, and the like.

While embodiments discussed above describe exemplary implementations ofcomponents within an equipment rack or CDU, one or more of theprinciples, aspects, or features described above may be used in otherapplications. For example, generation of power metrics as describedabove, as well as internal clocking based on an incoming AC signal, maybe incorporated in or with a power supply, such as a switched-mode powersupply, to provide metrics related to the power supply or to otherwiseuse them or the underlying operation or information monitoring inassociation with the power supply or associated components or systems.For example, in this fashion such a power supply may monitor itself,take corrective or other action based on (in whole or in part) internalmonitoring, and/or report out one or more power metrics. Such metricsmay be used, for example, to anticipate power supply failure, measurepower supply efficiency, and/or adjust the power supply to be moreefficient for a given load.

With reference now to FIG. 17, an embodiment illustrating powermonitoring incorporated within a switch mode power supply isillustrated. In this embodiment, a switch mode power supply 3000receives incoming AC power from an AC line source 3010. This embodimentincludes voltage and current monitoring for both the high side, that isthe high voltage AC input power, and the low side that is the relativelylow voltage DC output from the switch mode power supply 3000. The switchmode power supply 3000 is used to provide power to a load 3020, whichmay be any device or asset that receives power from the switch modepower supply 3000. The load 3020 is modeled as a resistive load in thisillustration, although it will be readily recognized that such loads arenot necessarily purely resistive loads, and in many cases if the load isoperating at less than optimal conditions, the load 3020 may be areactive load or have a larger reactive component relative to a loadoperating at optimal conditions.

A microcontroller 3030 receives an input from a toroidal currenttransformer 3040 associated with the high side AC power source. Theoutput of the current transformer 3040 indicates the instantaneousmagnitude of the current that is flowing through the input AC line, andmay be configured such as the current transformers described above. Theoutput of a voltage sense circuit 3050 is also received at themicrocontroller 3030. The voltage sense circuit 3050 may include anisolating amplifier that amplifies voltage from a voltage dividernetwork 3060, and may also include a frequency sense output such asdescribed above.

The microcontroller 3030 of this embodiment also receives input relatedto low side current and voltage. Current from the low side may be inputthrough a shunt resistor 3060 having a known resistance, the voltageacross this shunt resistor 3060 used to calculate the current providedto the load 3020. Low side voltage is provided from a voltage dividernetwork 3070. It is noted that the low side current and voltage sensesignals are not isolated signals, as these signals in this embodimenthave relatively low voltage levels that do not require isolation. Itwill be understood that necessary isolation may be achieved according tomany common methods. The microcontroller 3030 operates to collectinformation related to the voltage and current inputs and may processand output information in manners such as described above to providepower metrics related to the switch mode power supply 3000. The outputfrom the microcontroller 3030 may be through a communications buss 3080as illustrated in FIG. 17, although other communication may be utilizedsuch as wireless communications. The microcontroller 3030 of thisembodiment also provides a control output 3090 that may be used tocontrol one or more other components associated with the switch modepower supply 3000.

For example, typical power supplies are most efficient, when in goodoperating order, at a load of 80-90% of standard capacity. In the eventthat a power supply load is only 60% of capacity, and the load appearsstatic, the power supply could itself adjust internally, based on theload, to be more efficient. Embodiments such as described above canprovide the metrics or underlying measurements (e.g., waveformcomparisons) to trigger the adjustment. The power supply can alsoinclude a remote reporting capability to report out information.

Additionally, in certain other embodiments, generation of power metricsas described above and internal clocking based on an incoming AC signal,are incorporated into other types of appliances other thancomputing-related equipment, such as household computer, TV, stereo,and/or other appliances. Such appliances may use the information toadjust internally based on load and/or report out problems, powermetrics, etc. Such communications may be through a wired or wirelesscommunications interface to a remote power manager interconnected, forexample, to the smart grid. In some embodiments, the power supplycalculates only some, or none of the above noted metrics, but uses thistype of monitoring to take action.

As shown in FIG. 18, in some embodiments a power distribution unit 1801is adapted for vertical mounting in an equipment rack 1803. The rack1803 has a first back panel 1805 and a second back panel 1807 shown inopen position revealing the interior of the rack as seen from the rear.The unit 1801 is installed vertically against a side panel 1809. In therack shown, the side panel 1809 includes an upper horizontal member1811, a middle horizontal member 1813 and a lower horizontal member1815, and the unit 1801 is mounted to one or more of these horizontalmembers. For example, an upper bracket 1817 connects an upper extremity1819 of the unit 1801 to the upper horizontal member 1811 and a lowerbracket 1821 connects a lower extremity 1823 of the unit 1801 to thelower horizontal member 1815. First and second power inputs 1825 and1827 are positioned out of the way of any electrical components that maybe installed in the rack, penetrating the lower extremity 1823, althoughthese power inputs could be located elsewhere as convenient.

In some embodiments a power distribution unit is adapted for horizontalmounting using appropriate hardware for attachment to a horizontalsurface, for example an underside of an upper panel 1829 or between thelower horizontal member 1815 on one side of the rack and a correspondinghorizontal member (not shown) on the other side of the rack.

As shown in FIG. 19, in some embodiments a power distribution unithousing 65 is disposable in an electrical equipment rack of the type inwhich a plurality of electrical components are removably mountable. Asdiscussed previously, an example 1803 of such a rack is shown in FIG.18, although many other models of equipment rack might also be used. Theembodiment shown in FIG. 19 is in some respects similar to theembodiment shown in FIG. 2, and items in FIG. 19 that are similar toitems in FIG. 2 have like reference numerals and will not be furtherdiscussed. In the embodiment shown in FIG. 19, a power input 70penetrates the housing 65; a plurality of power outputs 202-216 aredisposed in the housing; a processor 1901 is disposed in the housing; avoltage calculation procedure 1903 is communicable with the processor; avoltage sensor (not shown) is communicable with the power input and theprocessor; and a plurality of current sensors (not shown) arecommunicable with the power outputs and the processor The voltage sensorand current sensors may be similar to the voltage sensor 52 and currentsensors 56, respectively, as shown in FIG. 1. The processor 1901 may beimplemented as a microprocessor, as an analog-to-digital converter andan arithmetic logic unit, or other devices. The voltage calculationprocedure may be implemented as computer instructions stored in memory,firmware, an application-specific integrated circuit (ASIC), or otherdevice. The voltage calculation procedure may for example be an RMScalculation procedure. In some embodiments the voltage calculationprocedure is disposed in the housing as shown, and in other embodimentsthe voltage calculation procedure is located elsewhere. Similarly, thevoltage and current sensors may be disposed in the housing or elsewhere.

In certain embodiments, assets that receive power from a PDU includepower supplies having such power measurement and reporting circuitry.The PDU includes a communication interface (wired or wireless) andreceives power supply metrics from each unit of supported electronicsequipment through the communications link. The PDU can utilize and/orreport the metrics to other remote entities.

The phrase “Per Outlet Power Sensing” (“POPS”) refers to the concept ofmonitoring power consumption at each outlet as discussed above. With anInternet interface, monitoring power consumption at each outlet providesdetailed power information and allows grouping of outlets to determinekilowatt consumption per device, group of devices, CDU, or cabinet.Power consumption can also be determined per rack, rows of racks, anentire data center, or the like by clustering outlet information acrossmultiple IP addresses and CDUs, as discussed above. This can provideconsolidated CDU information within a data center or across multiplelocations, a centralized location to view power and environmentalstatus, capacity planning, reports and trends, multiple views, autodiscovery of all CDU devices, alarm details, an ability to manage CDUs,global or individual outlet control, and logging.

In can thus be seen that embodiments provide a number of novel featuresand advantages including, for example: (a) sensing and output ofinformation related to the current and voltage output to variousdifferent components and/or applications; (b) an AC input clockcompensation solution integral to a microcontroller, in which a powermonitoring circuit and/or power meter does not require an externaloscillator for a time base; (c) predictive failure of various powercomponents; (d) an accurate energy accumulation scheme for one or moreoutputs associated with a single power monitoring and metering circuit;(d) output switching capability with relatively low power requirementsusing switching versus holding transistors in relay circuits used toswitch the outputs; (e) output switching at zero voltage crossings inthe AC power cycle; (f) modular construction of an outlet assembly withoptions to provide switched outputs or non-switched outputs; (g) theability to determine if lack of power at an outlet is the result of lossof input power or a blown fuse and (h) the ability to assess the healthof power supplies in an installed base of power supplies in data centerequipment racks without requiring any modification of the powersupplies.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

We claim:
 1. A power distribution unit comprising: a power distributionunit housing disposable in an electrical equipment rack of the type inwhich a plurality of electrical components are removably mountable; apower input penetrating the housing; a voltage sensor; a plurality ofpower outputs disposed in the housing, each in electrical communicationwith the power input and each connectable to an electrical load; aplurality of current sensors each in electrical communication with oneof the power outputs; and a power monitoring section in electricalcommunication with the voltage sensor and the current sensors andresponsive to signals provided by the sensors to (1) sample the signalsto obtain samples of voltage and current during one cycle of inputpower, (2) calculate raw RMS values of voltage and current from thesamples, and (3) scale the raw RMS values according to a predeterminedcalibration factor to obtain corrected RMS voltage and current valuesfor the load connected to each power output.
 2. A power distributionunit as in claim 1 wherein the power distribution unit housing isadapted for mounting within an electrical equipment rack in a verticalorientation.
 3. A power distribution unit as in claim 1 and furthercomprising a numeric display.
 4. A power distribution unit as in claim 3wherein the voltage sensor, the current sensors, the numeric display andthe power monitoring section are disposed in the power distribution unithousing.
 5. A power distribution unit as in claim 1 wherein the powermonitoring section stores the samples of voltage and current.
 6. A powerdistribution unit as in claim 1 wherein the power monitoring sectionstores the corrected RMS voltage and current values.
 7. A powerdistribution unit as in claim 1 wherein the power monitoring sectioncalculates for each load at least one of average active power, averageapparent power, power factor, and input current crest factor.
 8. A powerdistribution unit as in claim 1 wherein the power monitoring sectioncalculates a phase angle between the voltage across and the currentflowing through each load.
 9. A power distribution unit as in claim 1wherein the power monitoring section calculates energy consumed over aninterval longer than one cycle of input power by each load.
 10. A powerdistribution unit as in claim 1 wherein the power monitoring sectioncomprises an analog-to-digital converter (ADC) in electricalcommunication with the voltage sensor and the current sensors,processing logic in electrical communication with the ADC, and a memoryin electrical communication with the processing logic.
 11. A powerdistribution unit as in claim 10 wherein samples of voltage and currentobtained over more than one voltage cycle are stored in the memory. 12.A power distribution unit as in claim 11 wherein the processing logiccompares samples of voltage and current obtained with respect to one ofthe loads during a first voltage cycle with samples of voltage andcurrent obtained with respect to the same load during a second voltagecycle and generates an alert signal if there is a difference ofpredetermined magnitude therebetween.
 13. A power distribution unit asin claim 10 and further comprising, stored in the memory, model samplesrepresenting one of the loads.
 14. A power distribution unit as in claim13 wherein the processing logic compares the model samples with samplesobtained with respect to the same load during a voltage cycle andgenerates an alert signal if there is a difference of predeterminedmagnitude therebetween.
 15. A power distribution unit as in claim 1 andfurther comprising a timing sensor in electrical communication with thepower input and the power monitoring section.
 16. A power distributionunit as in claim 15 wherein the power monitoring section is responsiveto the timing sensor to calculate a frequency of the input power.
 17. Apower distribution unit as in claim 16 wherein the power monitoringsection uses the calculated frequency to compensate for errors in aninternal processor time base.
 18. A power distribution unit as in claim15 wherein the power monitoring section is responsive to the timingsensor to calculate a reload value and to use the reload value todetermine sampling intervals for sampling the voltage and current.
 19. Apower distribution unit as in claim 1 and further comprising acommunication section for establishing communication between the powermonitoring section and a remotely located network power manager.
 20. Apower distribution unit comprising: a power distribution unit housingdisposable in an electrical equipment rack of the type in which aplurality of electrical components are removably mountable; a powerinput penetrating the housing; a plurality of power outputs disposed inthe housing; a processor disposed in the housing; a voltage calculationprocedure communicable with the processor; a voltage sensor communicablewith the processor; and a plurality of current sensors each communicablewith one of the power outputs and with the processor.
 21. A powerdistribution unit as in claim 20 and further comprising a storage unitcommunicable with the processor.
 22. A power distribution unit as inclaim 20 wherein the power distribution unit housing is adapted formounting within an electrical equipment rack in a vertical orientation.23. A power distribution unit as in claim 20 and further comprising anumeric display.
 24. A power distribution unit as in claim 23 whereinthe voltage sensor, the current sensors, the numeric display and thevoltage calculation procedure are disposed in the power distributionunit housing.
 25. A power distribution unit as in claim 20 and furthercomprising at least one of an RMS calculation procedure, an averageactive power calculation procedure, an average apparent powercalculation procedure, a power factor calculation procedure, an inputcurrent crest factor calculation procedure, a voltage-to-current phaseangle calculation procedure, and an energy usage calculation procedurecommunicable with the processor.
 26. A power distribution unit as inclaim 20 wherein the processor comprises an analog-to-digital converterand an arithmetic logic unit.
 27. A power distribution unit as in claim20 and further comprising an historical comparison procedurecommunicable with the processor.
 28. A power distribution unit as inclaim 20 and further comprising an ideal-model comparison procedurecommunicable with the processor.
 29. A power distribution unit as inclaim 20 and further comprising a timing sensor communicable with thepower input.
 30. A power distribution unit as in claim 20 and furthercomprising a communication section communicable with the processor andwith a communication network external of the housing.
 31. A method ofmanaging a plurality of electrical loads each drawing electrical powerfrom a power distribution unit, the method comprising: sampling voltageacross and current flowing through each of the loads repeatedly duringone cycle of input power; calculating raw RMS values of voltage andcurrent from the samples; and scaling the raw RMS values according to apredetermined calibration factor to obtain corrected RMS voltage andcurrent values for each of the loads.
 32. A method as in claim 31 andfurther comprising storing the samples of voltage and current.
 33. Amethod as in claim 32 wherein storing the samples comprises storingsamples of voltage and current obtained over more than one voltagecycle.
 34. A method as in claim 31 and further comprising storing thecorrected RMS voltage and current values.
 35. A method as in claim 31and further comprising calculating for each load at least one of averageactive power, average apparent power, power factor, input current crestfactor, phase angle between voltage across and current flowing throughthe load, and energy consumed over an interval longer than one cycle ofinput power by the load.
 36. A method as in claim 31 and furthercomprising: comparing samples of voltage and current obtained withrespect to one of the loads during a first voltage cycle with samples ofvoltage and current obtained with respect to the same load during asecond voltage cycle; and generating an alert signal if there is adifference of predetermined magnitude therebetween.
 37. A method as inclaim 31 and further comprising storing model samples representative ofone of the loads.
 38. A method as in claim 37 and further comprising:comparing the stored model samples with samples of voltage and currentobtained with respect to the same load during a voltage cycle; andgenerating an alert signal if there is a difference of predeterminedmagnitude therebetween.
 39. A method as in claim 31 and furthercomprising sensing a zero crossing of a cycle of input power.
 40. Amethod as in claim 39 and further comprising calculating a frequency ofthe input power.
 41. A method as in claim 40 and further comprisingcompensating an internal time base according to the frequency.
 42. Amethod as in claim 39 and further comprising calculating a reload value;and determining sampling intervals for sampling the voltage and currentaccording to the reload value.
 43. A method as in claim 31 and furthercomprising communicating the corrected RMS voltage and current values toa remotely located network power manager.
 44. A power distribution unitas in claim 1 wherein the power distribution unit housing is adapted formounting within an electrical equipment rack in a horizontalorientation.
 45. A power distribution unit as in claim 20 wherein thepower distribution unit housing is adapted for mounting within anelectrical equipment rack in a horizontal orientation.